Road surface friction coefficient estimating method, signal multiplex transmission method and signal multiplex transmission device

ABSTRACT

A method for directly and accurately estimating friction coefficient of a road surface independently from the slip rate is disclosed. The method measures tangential and vertical forces acting on an elastic body  3  of an elastic wheel  1 , and calculates the friction coefficient of a road surface based on the measured values of these forces and angular rate of the wheel  1.

TECHNICAL FIELD

The present invention relates to a method for accurately estimatingfriction coefficient of a road surface which coefficient isindispensable to a control for enhancing a performance of an anti-lockbrake system (herein after referred to as an “ABS”) or a tractioncontrol system.

BACKGROUND

In order to improve a performance of an ABS used in a vehicle, it isconsidered to be effective to control lock and unlock states in acondition where friction coefficient of a road surface be as large aspossible. The friction coefficient of the road surface depends on a sliprate of a tire/wheel assembly under a certain condition of the roadsurface and therefore the ABS is designed to control lock and unlockstates of braking near a slip rate providing the maximum frictioncoefficient of the road surface.

For this reason, it is a common practice for the conventional ABS tomeasure a speed of the vehicle and a revolution of the tire/wheelassembly, calculate the slip rate based on the measured values, andautomatically control the braking so as the slip rate to fall within acertain range.

However, the method for estimating the frictional coefficient of theroad surface from the slip rate has a problem in which the relationshipbetween the slip rate and the friction coefficient of the road surfacedrastically changes depending on a road surface condition to vary theslip rate corresponding to the optimum friction coefficient of the roadsurface depending on the road surface condition, so that the optimumfriction coefficient of the road surface cannot be obtained from theslip rate alone. Although approaches for solving this problem such asseparately estimating the road surface condition as well has been made,satisfactory means have not been proposed yet.

Meanwhile, in connection with this point, an approach for more directlymeasuring and estimating the friction coefficient of the road surface.Such an approach is known from the disclosure of Japanese PatentApplication Opened No. 06-288798A. According to this disclosure, astrain gauge is attached to a suspension suspending the tire/wheelassembly and a strain occurring on this is measured to give a componentparallel to the friction force of the road surface or a componentperpendicular to the former component of a force acting on thesuspension. The means estimate the friction coefficient of the roadsurface based on these measured values by assuming the values as thefriction force of the road surface and the vertical force, respectively.

However, although the method with using a strain gauge is a more directestimating method as compared with the method of estimating the frictioncoefficient with the slip rate, there are problems in which the point ofmeasuring the force is far from a vicinity of tire which is the actualpoint of action of the friction force, so that a measuring result havingbeen influenced by disturbances applied between the point of action ofthe friction force and the point of measuring the force is obtained, andthat the strain gauge is applied on the suspension in which a generatedstrain is small, and the generated strain is converted to the force, sothat its accuracy is not sufficient.

The present invention has been made in view of these problems. Itrelates to a method for directly estimating friction coefficient of aroad surface independently from the slip rate, and its object is toprovide a method of real-timely and more accurately estimating thefriction coefficient of the road surface by measuring the friction forceof the road surface and the vertical force at a region near the tire aswell as accurately measuring these forces, and a method and a device ofmultiplex transmission of signals upon transmitting a measurement signalof a force measured in the vicinity of tire to the ABS displaced on thevehicle body side.

DISCLOSURE OF THE INVENTION

The present invention has been completed to achieve the above-mentionedobject, and its gist, constitution and operation will be describedbelow.

(1)

A method of estimating friction coefficient of a road surface accordingto the present invention calculates, in a tire/wheel assembly having anaxle hub, a tire, and a wheel attached to the axial hub to support thetire, tangential and vertical components of a force acting between asection of a transmission path between the axle hub and the tire and theother section of the transmission path, based on measurement values ofrelative displacements in the tangential and vertical directions,respectively, between the sections of the transmission path, thesections of the transmission path being bounded by an elastic bodyarranged within the transmission path; and, in calculating the frictioncoefficient between the tire/wheel assembly and the road surface basedon the acting force of these components, an angular rate of thetire/wheel assembly is measured along with measurements of eachcomponent of the acting force, and the friction coefficient μ betweenthe tire/wheel assembly and the road surface is calculated from theequation (1) with the measured tangential force Fb, vertical force N andangular rate ω as well as the known moment of inertia IΩ and knowneffective radius R of the tire/wheel assembly.μ=(Fb+(I ₀ /R)dω/dt)/N  (1)

According to the method of estimating the friction coefficient of theroad surface, the friction coefficient is not calculated by measuringthe slip rate and indirectly determining the friction coefficient of theroad surface from the slip rate with a relational expression between theslip rate and the friction coefficient of the road surface, but it isdirectly calculated by measuring the tangential and vertical componentof the force acting on a point of the vehicle. Therefore, the method canaccurately and real-timely calculate the friction coefficient whichalways changes depending on a condition of the road surface, so that anABS superior in a braking performance can be provided.

Further, it measures a force acting at a point on the vehicle at a pointon the tire/wheel assembly being closest to a boundary between the roadsurface and the assembly at which the friction force actually acts, sothat the friction force can be more accurately determined, therebycontributing an improvement of a performance of the ABS.

(2)

Moreover, in the present invention, the friction coefficient μ iscalculated by the equation (1), so that it can be estimated with highaccuracy. A ground for this equation is discussed with reference to FIG.10. FIG. 10 is a side view of a wheel 1 and a tire 2 constituting a partof the tire/wheel assembly taken from a side. The tire/wheel assemblyrotates at the angular rate ω. When the brake is applied on thetire/wheel assembly, the braking force Fb due to a braking torque Tb,the friction force Fd with respect to the rotation, and a vertical forceN act on a part P displaced in a place at a distance of an effectiveradius R of the tire/wheel assembly from the rotational center.Regarding positive and negative directions of the force, the directionas shown in FIG. 10 by an arrow is defined a positive direction. Thedynamic equation of this part can be expressed by the equation (2) withthe moment of inertia being I₀. The relationship between the brakingtorque Tb and the braking force Fb can be expressed by the equation (3),and the relationship between the friction force and the vertical forceby the equation (4). In the equation (2), when the actual measured valueof the tangential force is converted to the braking torque, a radius rof the elastic body from the center of the tire/wheel assembly isassumed to be the same as the effective radius R of the tire/wheelassembly. However, if the radius r is different from the radius R, themeasured tangential force may be multiplied by (r/R) to obtain thebraking force.I ₀·(dω/dt)=R·Fd−Tb  (2)Tb=R·Fb  (3)Fd=μ·N  (4)These equation (2), (3), and (4) can derive the equation (1).(2)

The method of estimating the friction coefficient of the road surfaceaccording to the present invention is based on the method described in(1), wherein the tangential relative displacements are summed among eachof four pairs of corresponding points comprising four pointssymmetrically arranged on the section of the transmission path about itsaxial center and, associated with these points, four correspondingpoints symmetrically arranged on the other section of the transmissionpath about its axial center, and the tangential relative displacement ofthe sections of the transmission path is measured based on the summedvalue.

By means of the method of estimating the friction coefficient of theroad surface, the tangential relative displacements are summed amongeach of four pairs of corresponding points to calculate the tangentialrelative displacements of the sections of the transmission path, so thatthey can be calculated independently from the position of the point ofmeasurement even not in a steady state.

This is discussed with reference to FIG. 11. FIG. 11 is a plane viewshowing a disk 30 as an example of one section of the transmission path,and a rim 31 as an example of the other section of the transmission pathbeing bounded by the elastic body. The rim 31 is connected with the disk30 via the not-shown elastic body. In a state where an eccentric loaddoes not exist, the rim 31 rotates along a rotational direction Q aboutan axial center identical to an axial center O₁ of the disk 30 inconjunction with the disk 30. In FIG. 11, an axial center O₂ of the rim31 is shifted by an amount of eccentricity D due to a vertical reactionforce T from the road surface.

In this method, one point P₁ of four points of measurement symmetricallydisplaced about the axial center O₁ of the disk 30 and one point P₂ offour points of measurement symmetrically displaced about the axialcenter O₂ of the rim 31 are made a pair, and then the tangentialcomponents of the displacement of each of the corresponding points P₁and P₂ of the pair are measured. Thereafter, the tangentialdisplacements of each of the corresponding points is similarlycalculated for the other not-shown three pairs.

All the point of measurement is displaced at a distance of a radius Rfrom each axial center, and rotates about each axial center. Themomentary rotation angle is designated as θ.

With respect to the shown pair of the points of measurement, thetangential displacement X₁ is calculated. When the disk 30 and the rim31 are not eccentric and additionally the tangential relativedisplacement is zero, the point P₂ of measurement on the rim 31 agreeswith the point P₁. When the tangential relative displacement α betweenthe disk 30 and the rim 31 occurs while remaining in the state where noeccentric exists, the point of measurement on the rim 31 is moved to thepoint P₂. The amount of displacement between the corresponding points isequal to the distance between the points P₁ and P₂, so that thetangential component X₁ is equal to the distance between the points P₁and P₃, which is expressed by the equation (5) as evidenced by thefigure.X ₁ =r·sin(α)+D·cos(θ)  (5)

Although the tangential relative displacement α needs to be calculated,the momentarily-changing relative displacement α between the disk 30 andthe rim 31 cannot be calculated based on only one pair of thecorresponding points, as is apparent from the equation. It is becausethe X₁ depends on the rotation angle θ of the point of measurement.Consequently, the method of estimating the friction coefficient of theroad surface enables a calculation of the tangential relativedisplacement α independently of the rotation position of a pair of thepoint of measurement by summing each of the tangential displacements ofthe four pairs of the corresponding points, and calculating thetangential relative displacement a based on the summed value Y₁, asshown in the equation (6). $\begin{matrix}\begin{matrix}{Y_{1} = {{r \cdot {\sin(\alpha)}} + {D \cdot {\cos(\theta)}} +}} \\{{r \cdot {\sin(\alpha)}} + {D \cdot {\cos( {\theta + {\pi\text{/}2}} )}} +} \\{{r \cdot {\sin(\alpha)}} + {D \cdot {\cos( {\theta + \pi} )}} +} \\{{r \cdot {\sin(\alpha)}} + {D \cdot {\cos( {\theta + {3\pi\text{/}2}} )}}} \\{= {4{r \cdot \sin}\quad(\alpha)}}\end{matrix} & (6)\end{matrix}$(3)

The method of estimating the friction coefficient of the road surfaceaccording to the present invention is based on the method described in(1), wherein the radial relative displacements are measured among eachof four pairs of corresponding points comprising four pointssymmetrically arranged on the section of the transmission path about itsaxial center and, associated with these points, four correspondingpoints symmetrically arranged on the other section of the transmissionpath about its axial center, the products of two pairs of the relativedisplacements in a diagonal relation among the relative displacement arerespectively calculated and summed, and the vertical relativedisplacement of the sections of the transmission path is measured basedon the summed value.

By means of this method of estimating the friction coefficient of theroad surface, the products of the radial relative displacements of eachof the four corresponding points are calculated, two pairs of thecorresponding points in a diagonal relation among the relativedisplacements, the products of the two pairs are summed, and thevertical relative displacement of the above-mentioned section and theother section of the transmission path of the tire/wheel assembly beingbounded by the elastic body is decided based on the summed value, sothat the displacement can be calculated independently from the positionof the point of measurement even not in a steady state.

This is discussed with reference to FIG. 12. FIG. 12 is a planar viewshowing a disk 30 as an example of one section of the transmission path,and a rim 31 as an example of the other section of the transmission pathbeing bounded by the elastic body. In FIG. 12, the rim 31 is connectedto the disk 30 via the not-shown elastic body. In a state where aneccentric load does not exist, the rim 31 rotates along a rotationaldirection Q about an axial center identical to an axial center O₅ of thedisk 30 in conjunction with the disk 30. In FIG. 12, an axial center O₆of the rim 31 is shifted by an amount of eccentricity D due to avertical reaction force T from the road surface.

In this method, one point P₅ of four points of measurement symmetricallydisplaced about the axial center O₅ of the disk 30 and one point P₆ offour points of measurement symmetrically displaced about the axialcenter O₆ of the rim 31 are made a pair, and then the radial componentsof the displacement of each of the corresponding points P₅ and P₆ of thepair are measured. Thereafter, the radial displacements of each of thecorresponding points are similarly calculated for the other not-shownthree pairs.

Each of the points of measurement rotates about each axial center andits momentary rotation angle is designated as θ.

With respect to the shown pair of the points of measurement, the radialdisplacement X₂ is calculated. When the disk 30 and the rim 31 are noteccentric, the point P₆′ of measurement on the rim 31 agrees with thepoint P₅ and the displacement of them is zero.

When an eccentricity occurs in an amount of D, the point of measurementon the rim 31 is moved to the point P₆. The amount of displacementbetween the corresponding points is equal to the distance between thepoints P₅ and P₆, so that the tangential component X₂ is equal to thedistance between the points P₅ and P₇ which is expressed by the equation(7) as evidenced by the figure.X ₂ =D·sin(θ)  (7)

Although the amount of eccentricity D needs to be calculated, themomentarily-changing amount of eccentricity D cannot be calculated basedon only one pair of the corresponding points, as is apparent from theequation. It is because the X₂ depends on the rotation angle θ of thepoint of measurement. Consequently, the method of estimating thefriction coefficient of the road surface enables a calculation of theamount of eccentricity D independently of the rotation position of apair of the point of measurement by measuring each of the radialdisplacements of the four pairs of the corresponding points, calculatingthe products of the two relative displacements in a diagonal relationamong the relative displacements, and calculating the amount ofeccentricity D based on the summed value Y₂, as shown in the equation(8). $\begin{matrix}\begin{matrix}{{Y2} = {( {( {D \cdot {\sin(\theta)}} ) \cdot ( {D \cdot {\sin( {\theta + \pi} )}} )} ) +}} \\{( {( {D \cdot {\sin( {\theta + {\pi/2}} )}} ) \cdot ( {D \cdot {\sin( {\theta + {3{\pi/2}}} )}} )} )} \\{= {{- 2}D^{2}}}\end{matrix} & (8)\end{matrix}$(4)

The method of estimating the friction coefficient of the road surfaceaccording to the present invention is based on the method described inany one of (1) to (3), wherein a hall element is used to measure achange in magnetic flux density of the magnetic body, and the tangentialor radial relative displacement is detected based on the amount of thechange.

By means of this method of estimating the friction coefficient of theroad surface, a hall element is used to measure the change in magneticflux density of the magnetic body to detect the tangential or radialrelative displacement, so that the friction coefficient of the roadsurface can be readily and accurately measured.

(5)

The method of multiplex transmission of signals according to the presentinvention, upon transmitting signals of the measured values described inany one of (1) to (4) or other signals between the vehicle body side andthe rotating tire/wheel assembly side, a composite signal obtained by avoltage/frequency conversion of a plurality of signals is applied totransmitting coil while switching the signals to transmit to a receivingcoil by an electromagnetic induction, said plurality of signals havingdynamic ranges without overlapping with each other, a plurality ofsignals obtained by a frequency/voltage conversion of the compositesignal received by the receiving coil is processed to generate timingsignals with a plurality of given voltage levels as threshold, and saidcomposite signal obtained by the frequency/voltage conversion is sampledaccording to said timing signals to reproduce a plurality of theoriginal signals.

The above-mentioned measured values described in any one of (1) to (4)are measured by sensors provided on the rotating tire/wheel assemblyside. The measured values need to be transmitted at least to an ABSwhich controls a braking with using the measured values. Therefore, asignal transmittance between the rotating tire/wheel assembly side andthe vehicle body side provided with the ABS is needed.

For such a method of transmitting signals between a rotating part andnon-rotating part, a method of transmitting signals on carrier waves,so-called a wireless method and a wired method can be used. However, theformer has problems in a complicated configuration as well as a largeelectric power consumption, and the latter, which transmits the signalsbetween the rotating part and the non-rotating part via a mechanicalcontact point such as a slip ring so that the mechanical contact pointextremely deteriorates by wear, has a problem in a maintenance.

As the third method, there has been suggested a method of displacing atransmitting coil on the wheel of the tire side as well as displacing areceiving coil on the vehicle body side to face the transmitting coil,and transmitting a detecting signal from a sensor displaced on the tireside to the receiver coil by the action of electromagnetic induction.However, such a method of transmitting signals by the action ofelectromagnetic induction is not configured to multiplex a plurality ofsignals.

By means of the system of multiplex transmission of signals according tothe present invention, for example, when the above-mentioned relativedisplacements are measured while the vehicle is running, and themeasured signals are transmitted to the vehicle body side, the detectedsignals output from a plurality of sensors displaced on the tire sideare converted into signals having dynamic ranges without overlappingwith each other. Then, as mentioned above, a voltage/frequencyconversion is conducted to generate a composite signal which istransmitted from the transmitting coil to the receiving coil by theaction of electromagnetic induction. The composite signal received bythe receiving coil is subjected to the frequency/voltage conversion, anda plurality of timing signals are generated with a plurality of givenvoltage level as thresholds. The composite signal obtained by thefrequency/voltage conversion is sampled/held according to the timingsignals to be able to reproduce a plurality of the original signals.

(6)

The device of multiplex transmission of signals according to the presentinvention is used for the method of multiplex transmission of signalsdescribed in (5), which comprises:

a signal-generating circuit for generating a plurality of signals havingdynamic ranges without overlapping with each other; a switching circuitfor outputting a composite signal by sequentially switching theplurality of signals simultaneously output from the signal-generatingcircuit at a given cycle; a voltage/frequency conversion circuit forconverting the composite signal output from the switching circuit into acomposite signal having a frequency corresponding to the voltage; anoutput circuit for amplifying the composite signal output from thevoltage/frequency conversion circuit; a transmitter coil fortransmitting the amplified composite signal output from the outputcircuit; a receiver coil for receiving the composite signal transmittedfrom the transmitter coil by the action of electromagnetic induction; afrequency/voltage conversion circuit for converting the composite signalreceived by the receiver coil into a composite signal having anamplitude corresponding to the frequency; a timing signal-generatingcircuit for generating a plurality of timing signals by processing thecomposite signal output from the frequency/voltage conversion circuitwith a plurality of given voltage levels as thresholds; a plurality ofsample/hold circuits for sampling the composite signal output from thefrequency/voltage conversion circuit according to the timing signals toreproduce a plurality of the original signals.

By means of the device of multiplex transmission of signals according tothe present invention, as is apparent from its configuration, aplurality of the signals described in (5) can be multiplexed and themultiplexed signals can be transmitted between the tire/wheel assemblyside and the vehicle body side.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of the tire/wheel assembly showing anembodiment of the measurement method according to the present invention;

FIG. 2 is a side view of the tire/wheel assembly showing in a statewhere the tire/wheel assembly is rotating;

FIG. 3 is a side view of the tire/wheel assembly taken in a direction ofthe arrows III—III in FIG. 1;

FIG. 4 is a configuration view showing a configuration of the tangentialand vertical displacement sensor bodies;

FIG. 5 is a block diagram of the circuit for computing the output of thetangential displacement sensor body;

FIG. 6 is a block diagram of the circuit for computing the output of thevertical displacement sensor body;

FIG. 7 is a block diagram showing the friction coefficient computingsection;

FIG. 8 is a block diagram showing the whole configuration of the signalmultiple signal transmitting device;

FIG. 9 is a signal wave form chart for explaining the generation of thetiming signals;

FIG. 10 is a side view of the wheel and the tire;

FIG. 11 is a plane view showing the disk and the rim connected thereto;

FIG. 12 is a plane view showing the disk and the rim connected thereto;

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to FIGS. 1 to 12. FIG. 1 is a sectional view of a wheel 1showing in a state where the wheel 1 is attached to an axle hub 7 of avehicle. The wheel 1 is provided with a disk 4 mounted to the axle hub7, a rim 2 supporting a tire 6, and a rubber-like elastic body 3connecting the disk 4 with the rim 2 and equipped in a part of aforce-transmission path between the axle hub 7 and the tire 6. Beingbounded by the rubber-like elastic body 3, one section of theforce-transmission path is composed of the disk 4, and the other sectionof the force-transmission path is composed of the rim 2. A tire/wheelassembly is consist of the axle hub 7, the tire 6 and the wheel 1.

FIG. 2 is a side view of the wheel showing in a state where the wheel 1is mounted on a running vehicle and rolls along the road surface. Therim 2 connected to the disk 4 via the rubber-like elastic body 3 isnearly concentric with the axial line of the disk 4. As the rubber-likeelastic body 3 deforms in the vertical direction due to a verticalaction of the gravity, the rim 2 is relatively displaced by an amount ofeccentricity D in a direction T with respect to the disk 4. The rim 2 isdriven via the rubber-like elastic body 3 by the disk 4 to rotate in arotational direction Q. In this context, the rim 2 torsionally rotateswith a tangential relative displacement α with respect to the disk 4 dueto an elastic deformation of the rubber-like elastic body 3 in therotational direction.

FIG. 3 is a side view of the wheel taken in a direction of the arrowsIII—III in FIG. 1. As shown in FIG. 3, tangential displacement sensorbodies 11 and vertical displacement sensor bodies 12 are provided atfour points on a periphery of the boundary between the rim 2 and disk 4of the wheel 1.

FIG. 4 is a configuration view showing the configuration of thetangential displacement sensor body 11 and the vertical displacementsensor body 12. These sensor bodies 11 and 12 are composed of a barmagnet 15, a hall element probe 14 oppositely disposed on an axial lineL of the bar magnet 15, and an amplifier 16. These sensor bodies 11 and12 can detect changes in a gap between the tip of bar magnet 15 and thetip of the hall element probe 14 along the axial line.

Four bar magnets 15 constituting the four tangential displacement sensorbodies 11 are attached on the rim 2 to positions being symmetric withrespect to the axial center of the rim 2. Four hall element probes 14each corresponding to respective bar magnet 15 are attached on the disk4 to positions being symmetric with respect to the axial center of thedisk 4. The axial line L is directed to the tangential line.

In this state, each pair composed of the tip of the bar magnet 15 andthe tip of the corresponding hall element probe 14 constitutes a pairedcorresponding point. Tangential relative displacements of thesecorresponding points are measured and the relative displacements ofthese four pairs are summed. A tangential displacement of the disk 4with respect to the rim 2 is calculated based on the summed value Y₁.That is, when a continuously-changing rotation angle of the wheel 1, adistance from the axial center of the corresponding points, an amount ofeccentricity, and the tangential relative displacement between the rim 2and the disk 4 are designated as θ, r, D, and α, respectively, thetangential relative displacement a can be calculated from the summedvalue Y₁ of the four pairs of the relative displacements according tothe above-mentioned equation (6). The value Y1 is independent from thecontinuously-changing rotation angle θ, so that the tangential relativedisplacement a can be real-timely calculated.

FIG. 5 is a block diagram of a circuit for computing an output of thetangential displacement sensor body 11. The outputs of amplifiers 16A ofthe tangential displacement sensor body 11 are input to an accumulator21 to output a summed value Y₁.

In the next, a method of calculating a vertical displacement withrespect to the disk 4 of the rim 2, i.e. an amount of eccentricity isdescribed. Four bar magnets 15 constituting the four verticaldisplacement sensor bodies 12 are attached on the rim 2 to positionsbeing symmetric with respect to the axial center of the rim 2. Four hallelement probes 14 each corresponding to the respective bar magnets 15are attached on the disk 4 to positions being symmetric with respect tothe axial center of the disk 4. The axial line L is directed to theradial direction.

In this state, each pair composed of the tip of the bar magnet 15 andthe tip of the corresponding hall element probe 14 constitutes a pairedcorresponding point. Radial relative displacements of thesecorresponding points are measured and the products of two pairs of therelative displacements in diagonal relationships among the relativedisplacement are calculated. The two pairs of the products are summed tocalculate the amount of eccentricity based on the summed value Y₂. Thatis, when a continuously-changing rotation angle of the wheel 1 and theamount of eccentricity are designated as θ and D, respectively, theamount of eccentricity D can be calculated from the summed value Y₂according to the above-mentioned equation (6). The value Y₂ does notdepend on the continuously-changing rotation angle θ, so that that theamount of eccentricity D can be real-timely calculated as an amountindependent from the rotation angle θ.

FIG. 6 is a block diagram of a circuit for computing an output of thevertical displacement sensor body 12. The outputs from two multiplierfor multiplying the outputs of the amplifiers 16B of the verticaldisplacement sensor bodies 12 diagonally displaced each other are summedby the accumulator 24 to out put as Y₂. Y₂ is computed by a softwaresection 25 to inversely calculate a eccentricity value Y₃ correspondingto the amount of eccentricity D according to the equation (8).

FIG. 7 is a block diagram showing a friction coefficient-computingsection 26 for calculating the friction coefficient. The frictioncoefficient-computing section 26 estimates and calculates the frictioncoefficient based on the summed value Y₁ from the tangentialdisplacement sensor body 11, the amount of eccentricity Y₃ from thevertical displacement sensor body 12, and an input value Z of angularrate from a not-shown rotational speed sensor of the tire/wheel assemblyaccording to the method of the above-mentioned equation (1).

The method of estimating the friction coefficient based on the output ofeach displacement sensor body has been described in the above withreference to the block diagrams shown in FIGS. 5-7. The amplifiers 16Aand 16B of each displacement sensor body, accumulators 21 and 24constituting the computing circuits for calculating the summed values Y₁and Y₂, and the multiplier 22 are attached to each of the rotatingtire/wheel assemblies and a computer section for calculating thefriction coefficient from the summed values Y₁ and Y₂, i.e. the softwaresection 25 and the friction coefficient-computing section 26 areequipped on the vehicle body side, so that the summed values Y₁ and Y₂have to be transmitted from the tire/wheel assembly side to the vehiclebody side, which is preferably not by means of signal lines. Because awired transmission can transmit the signals via an slip ring or the likebut it cannot avoid problems resulting from a wear or a heat due to aslip between the rotating section and the fixed section.

When the transmission is not by means of signal lines, it is wasteful toprovide two transmission paths and transmit the summed values Y₁ and Y₂from the tire/wheel assembly side to the vehicle body side by the paths,so that a multiplex transmission is preferable. Although transmittingsignals by means of a radio wave is a common method among the methodsnot by means of signal lines, such a signal transmission from thetire/wheel assembly side to the vehicle body side of the running vehicletends to be affected by a noise, so that a system of transmitting asignal by an electromagnetic induction is adopted.

With reference to FIGS. 8 and 9, a multiplex transmitting device used inthis embodiment is described below. FIG. 8 is a block diagram showing awhole configuration of the multiplex transmitting device 40. Themultiplex transmitting device 40 transmits the summed value signals Y₁and Y₂ from the tire/wheel assembly side to the vehicle body side. Themethod of transmitting signals from the tire/wheel assembly side to thevehicle body side is discussed below. Original signals consisting of thesummed values Y₁ and Y₂ are input to a signal generating circuit 29 andare converted at the signal generating circuit 29 without overlappingdynamic ranges with each over. As an example of the dynamic range, forexample, the summed values Y₁ are in the voltage range of 0.5-1.0 V andY₂ are in voltage ranges of 1.5-2.0V.

Then, the converted summed value signals are supplied to a switchingcircuit 32 and the switching circuit 32 sequentially switches andoutputs the signals at a given cycle. In this specification, such asignal output from the switching circuit 32 is referred to as a“composite signal”. As the dynamic ranges of the summed value signals Y₁and Y₂ are different from each other, the signal jumps at the time ofswitching by the switching circuit 32.

The composite signal output from the switching circuit 32 in this manneris then supplied to a voltage-frequency conversion circuit 33. Thevoltage-frequency conversion circuit 33 has a function of converting theinput signal to the output signal with the frequency corresponding thevoltage, so that the voltage-frequency conversion circuit 33 outputs thecomposite signals with their frequency ranges being sequentiallychanged. Further, after the composite signals output from thevoltage-frequency conversion circuit 33 are amplified by the outputcircuit 34, the amplified composite signal output from the outputcircuit 34 is supplied to a transmitting coil 35. The transmitting coil35 is displaced to be concentric with the axle hub 7 and rotatesintegrally with the tire 6 and the wheel 1.

On the vehicle body side of the vehicle, a receiving coil 36 is disposedto face the above-mentioned transmitting coil 35 and is configured toreceive the composite signals transmitted by the electromagneticinduction from the transmitting coil 35. As mentioned above, theamplifiers 16A and 16B of each displacement sensor body, the computingcircuit including the accumulators 22 and 24, and multiplier 23, theswitching circuit 32, the voltage-frequency conversion circuit 33, andthe output circuit 34 as well as the transmitting coil 35 are disposedon the tire/wheel assembly side. Electric power for them is supplied bythe electromagnetic induction from the receiving coil 36 to thetransmitting coil 35, so that a power source such as a battery is notneeded. As supplying electric power by the electromagnetic induction ofthe coil is known per se, no further detail is discussed here.

As mentioned above, the composite signal received by the receiving coil36 is supplied to the frequency/voltage conversion circuit 37 to convertit to a composite signal having an amplitude corresponding to thefrequency. The composite signal output from the frequency/voltageconversion circuit 37 contains mixed information of multiple signals, sothat the signals need to be identified. To this end, the compositesignal output from the frequency/voltage conversion circuit 37 issupplied to a timing signal generating circuit 38. The timing signalgenerating circuit 38 sets a plurality of given voltage levels as thethresholds. As shown in FIG. 9, the voltage at each boundary of thedynamic ranges, e.g. 1.2 V is set as the threshold.

In the case where the first signal S₁ in the composite signal switchesto the second signal, the amplitude of the composite signal goes acrossthe threshold of 1.2 V, and in the case where the second signal S₂ inthe composite signal switches to the first signal, the amplitude of thecomposite signal goes across inversely (from the larger region to thesmaller region). In this way, the amplitude of the composite signal goesacross sequential thresholds at the switching of the sequential signalscontained in the composite signal, so that the given signal can beappropriately sampled and held by sampling the signal at a time of thegiven time τ having passed since each of the timing signals wasgenerated from the timing signal generating circuit 38 at these timing.

In this manner, the plurality of the timing signals output form thetiming signal generating circuit 38 are respectively supplied to twosample/hold circuits 39 each receiving the composite signal output fromthe above-mentioned frequency/voltage conversion circuit 37. In thesesample/hold circuit, the composite signal is sampled and held accordingto the sampling signals generated at a time of the time X having passedsince each of the timing signals has generated, so that each of theoriginal signals contained in the composite signal can be reproduced. Inthis way, a plurality of signals can be multiplex by the electromagneticinduction with the transmitting coil 35 and receiving coil 36. Thesesignals are supplied to a signal processing circuit equipped in thevehicle body side where a given process is conducted to be able toestimate the friction coefficient.

In the above, a description is made to a case where the accumulators 21,24 and the multiplier 22 are equipped on the rotating tire/wheelassembly. However, the accumulator 21, 24 and the multiplier 24 may beequipped on the vehicle body side. In this case, total of eight signalsfrom the amplifiers 16A, 16B are input to the signal generating circuit29 and the eight kinds of signals are converted into eight signalshaving dynamic ranges different from each other. Thereafter, the signalsare transmitted to the vehicle body side as mentioned above to reproducethe eight original signals.

Moreover, as mentioned above, in the method and device of multiplextransmission of the signals according to the present invention,operations similar to the sampling of each signal are conducted at theswitching circuit 22, so that a switching frequency in the switchingcircuit 22 can be determined based on the sampling theorem. For example,when the highest frequency in a plurality of the signals to betransmitted and the number of the signals are designated as f_(H) and N,respectively, and each signal is transmitted at the same rate, theswitching frequency f_(S) may be f_(S)=2Nf_(H).

Further, the present invention is not limited to the above-mentionedexamples, there may be a number of modifications and variations. Forexample, in the above-mentioned example, the contiguous dynamic rangesof a plurality of the signals to be transmitted are set, butdiscontiguous dynamic ranges such as 0-1 V for the first signal and 2-3Vfor the second signal may be set. Also, in the above-mentioned example,all the transmitting rate of the signals are set to be equal bysequentially transmitting a plurality of the signals, but thetransmitting rate of each signal may be varied.

INDUSTRIAL APPLICABILITY

As having been clearly shown in the above description, according to thepresent invention, the tangential and vertical forces acting on theelastic body on the elastic wheel are measured and the frictioncoefficient of the road surface is estimated based on the measuredforces, so that the friction coefficient of the road surface can bedirectly and accurately estimated while being independent from the sliprate, which contributing an improvement in a performance of the ABS.

Moreover, when the measured values measured at the tire/wheel assemblyside are transmitted to the ABS on the vehicle body side, a plurality ofsignals are multiplexed and transmitted by the action of electromagneticinduction. Thus, a transmitting device being simple and highly tolerantof noises can be configured. In addition, by multiplexing the signals,the ABS capable of transmitting a number of information within a giventime and having a short response time can be realized.

Further, the method and device of multiplex transmission of signals areapplicable not only for the transmission between the rotating tire/wheelassembly and the vehicle body, but also for other application oftransmitting a number of signals without using signal lines.

1. A method for estimating friction coefficient of a road surface,comprising calculating, in a tire/wheel assembly having an axle hub, atire, and a wheel attached to the axial hub to support the tire,tangential and vertical components of a force acting between a sectionof a transmission path between the axle hub and the tire and the othersection of the transmission path based on measurement values of relativedisplacements in the tangential and vertical directions, respectively,between the sections of the transmission path, the sections of thetransmission path being bounded by an elastic body arranged within thetransmission path; and in order to calculate the friction coefficientbetween the tire/wheel assembly and the road surface based on the actingforce of these components, measuring an angular rate of the tire/wheelassembly along with measurements of each component of the acting force,and calculating the friction coefficient μ between the tire/wheelassembly and the road surface from the equation μ=(Fb+(I₀/R)dω/dt)/N,with the measured tangential force Fb, vertical force N and angular rateco as well as the known moment of inertia I₀ and known effective radiusR of the tire/wheel assembly.
 2. The method for estimating frictioncoefficient of a road surface according to claim 1, wherein thetangential relative displacements are summed among each of four pairs ofpoints comprising four points symmetrically arranged on said section ofthe transmission path about its axial center and, associated with thesepoints, four corresponding points symmetrically arranged on said theother section of the transmission path about its axial center, and thetangential relative displacement of said section and said the othersection of the transmission path is determined based on the summedvalue.
 3. The method for estimating friction coefficient of a roadsurface according to claim 1, wherein the radial relative displacementsare measured among each of four pairs of points comprising four pointssymmetrically arranged on said section of the transmission path aboutits axial center and, associated with these points, four correspondingpoints symmetrically arranged on said the other section of thetransmission path about its axial center, the products of two pairs ofthe relative displacements in a diagonal relation among the relativedisplacement are respectively calculated and summed, and the verticalrelative displacement of said section and said the other section of thetransmission path is determined based on the summed value.
 4. The methodfor estimating friction coefficient of a road surface according to claim1, wherein a hall element is used to measure a change in magnetic fluxdensity of a magnetic body, and the tangential or radial relativedisplacement is detected based on the amount of the change.
 5. A methodof a multiplex transmission of signals, characterized in that, upontransmitting signals of the measured values recited in claim 1, or othersignals, between the vehicle body side and the rotating tire/wheelassembly side, a composite signal obtained by a voltage/frequencyconversion of a plurality of signals is supplied to a transmitting coilwhile switching the signals to transmit to a receiving coil by anelectromagnetic induction, said plurality of signals having dynamicranges without overlapping with each other, a plurality of signalsobtained by a frequency/voltage conversion of the composite signalreceived by the receiving coil is processed to generate timing signalswith a plurality of given voltage levels as thresholds, and saidcomposite signal obtained by the frequency/voltage conversion is sampledaccording to said timing signals to reproduce a plurality of theoriginal signals.
 6. A device of multiplex transmission of signals forcarrying out the method of multiplex transmission of signals accordingto claim 5, comprising: a signal-generating circuit for generating aplurality of signals having dynamic ranges without overlapping with eachother; a switching circuit for outputting a composite signal bysequentially switching the plurality of signals simultaneously outputfrom the signal-generating circuit at a given cycle; a voltage/frequencyconversion circuit for converting the composite signal output from theswitching circuit into a composite signal having a frequencycorresponding to the voltage; an output circuit for amplifying thecomposite signal output from the voltage/frequency conversion circuit; atransmitter coil for transmitting the amplified composite signal outputfrom the output circuit; a receiver coil for receiving the compositesignal transmitted from the transmitter coil by the action ofelectromagnetic induction; a frequency/voltage conversion circuit forconverting the composite signal received by the receiver coil into acomposite signal having an amplitude corresponding to the frequency; atiming signal-generating circuit for generating a plurality of timingsignals by processing the composite signal output from thefrequency/voltage conversion circuit with a plurality of given voltagelevels as thresholds; a plurality of sample/hold circuits for samplingthe composite signal output from the frequency/voltage conversioncircuit according to the timing signals to reproduce a plurality of theoriginal signals.
 7. The method for estimating friction coefficient of aroad surface according to claim 2, wherein a hall element is used tomeasure a change in magnetic flux density of a magnetic body, and thetangential or radial relative displacement is detected based on theamount of the change.
 8. The method for estimating friction coefficientof a road surface according to claim 3, wherein a hall element is usedto measure a change in magnetic flux density of a magnetic body, and thetangential or radial relative displacement is detected based on theamount of the change.
 9. A method of a multiplex transmission ofsignals, characterized in that, upon transmitting signals of themeasured values recited in claim 2, or other signals, between thevehicle body side and the rotating tire/wheel assembly side, a compositesignal obtained by a voltage/frequency conversion of a plurality ofsignals is supplied to a transmitting coil while switching the signalsto transmit to a receiving coil by an electromagnetic induction, saidplurality of signals having dynamic ranges without overlapping with eachother, a plurality of signals obtained by a frequency/voltage conversionof the composite signal received by the receiving coil is processed togenerate timing signals with a plurality of given voltage levels asthresholds, and said composite signal obtained by the frequency/voltageconversion is sampled according to said timing signals to reproduce aplurality of the original signals.
 10. A method of a multiplextransmission of signals, characterized in that, upon transmittingsignals of the measured values recited in claim 3, or other signals,between the vehicle body side and the rotating tire/wheel assembly side,a composite signal obtained by a voltage/frequency conversion of aplurality of signals is supplied to a transmitting coil while switchingthe signals to transmit to a receiving coil by an electromagneticinduction, said plurality of signals having dynamic ranges withoutoverlapping with each other, a plurality of signals obtained by afrequency/voltage conversion of the composite signal received by thereceiving coil is processed to generate timing signals with a pluralityof given voltage levels as thresholds, and said composite signalobtained by the frequency/voltage conversion is sampled according tosaid timing signals to reproduce a plurality of the original signals.11. A method of a multiplex transmission of signals, characterized inthat, upon transmitting signals of the measured values recited in claim4, or other signals, between the vehicle body side and the rotatingtire/wheel assembly side, a composite signal obtained by avoltage/frequency conversion of a plurality of signals is supplied to atransmitting coil while switching the signals to transmit to a receivingcoil by an electromagnetic induction, said plurality of signals havingdynamic ranges without overlapping with each other, a plurality ofsignals obtained by a frequency/voltage conversion of the compositesignal received by the receiving coil is processed to generate timingsignals with a plurality of given voltage levels as thresholds, and saidcomposite signal obtained by the frequency/voltage conversion is sampledaccording to said timing signals to reproduce a plurality of theoriginal signals.